1. Field of the Invention
The invention relates in general to protective relay apparatus for electrical power systems, and more specifically to protective relay apparatus for detecting and clearing single-phase-to-ground faults, regardless of fault impedance.
2. Description of the Prior Art
A conventional distribution substation for supplying three-phase and single-phase loads includes a three-phase circuit breaker equipped with phase overcurrent relays, and sometimes a ground-current relay. Reclosers and fuses complete the protection of the distribution circuit. Coordination is necessary between these protective devices in order to isolate only the required portion of the distribution circuit when a fault occurs. Because of normal circuit load unbalances, as well as normal cold-load pickup and switching surges, a ground relay, if used, must be set quite high in order to prevent nuisance tripping of the circuit breaker. Broken conductors close to or touching the ground through high impedance paths may not produce enough current or circuit unbalance to operate the phase overcurrent protective devices, or the ground relay.
Co-pending application Ser. No. 245,768, filed Mar. 20, 1981, entitled "Ratio Ground Relays", by Warren J. Schmidt, which is assigned to the same assignee as the present application, discloses a new and improved ratio ground relay which will detect such high impedance single-phase-to-ground faults. This ratio ground relay is an electromechanical relay of the induction disc type having an "operate" electromagnet which provides an operating torque as a function of the zero sequence current 3I.sub.0, and a "restraint" electromagnetic which provides a restraint torque as a function of the positive sequence current I.sub.1 minus the negative sequence current I.sub.2. The restraint electromagnet includes a three-legged magnetic core structure, with a winding on each leg energized by a different phase current, which arrangement provides the I.sub.1 -I.sub.2 characteristic.